PHANEROGAMIA : AXIAL ORGANS. 239 



That we only speak of medullary rays in the case of Dicotyledons arises 

 from mere want of exactness of language, since the cellular tissue between 

 the vascular bundles of the Monocotyledons is just as much a medullary 

 ray as between the vascular bundles of the Dicotyledons, and as little 

 changed in its cellular formation as in those Dicotyledons where the 

 vascular bundles are very far removed from one another. Moreover, the 

 cells in highly compressed vascular bundles, especially in the external 

 parts of the stem of the Monocotyledons having a cambium circle, assume 

 precisely the same form and arrangement as the medullary cells ranged 

 in radial horizontal rows in the Dicotyledons, as, for instance, in the stem 

 of Aletris fret grans. 



We are able to assert very little generally concerning the structure of 

 the bark, since nothing is unconditionally true, with the exception of the 

 foundation being always composed of cellular tissue. No combination of 

 definite forms of the elementary organs is peculiar to all barks ; some 

 forms occur, however, so frequently, that it would appear desirable to 

 draw attention to them. Here I must, however, distinguish between 

 Monocotyledons and Dicotyledons. 



A. Monocotyledons. I am unable, from deficiency of a sufficient 

 number of investigations, to say anything important of the structural 

 relations of this group. As far as I know, the bark constantly consists 

 exclusively of parenchyma, which is more elongated in the interior than 

 towards the exterior having more chlorophyll towards the exterior, 

 but gradually losing it towards the interior, so that the cortical paren- 

 chyma constantly merges into the pith, wherever there is no sharp line 

 drawn by the formation of a wholly closed circle of strongly thickened 

 parenchymatous cells, which connects a ring of vascular bundles, as, for 

 instance, in Pothos. According to Mohl*, most Palms have a peculiar 

 layer, varying in thickness at different times, composed of thick- walled 

 parenchymatous cells, placed immediately under the epidermis. In 

 Grasses and the Cyperacece we find immediately below the epidermis 

 separate bundles of liber- cells. The cells of the epidermis above these 

 generally continue to have thin walls ; whilst in those parts where 

 parenchyma lies below, their walls become extremely thick, as, for 

 instance, in Papyrus antiquorum. 



B. Dicotyledons. 1. Annual Bark. In this we may, besides the 

 epidermis, distinguish three parts of the bark ; they do not, however, 

 constitute anything essentially characteristic of the bark, which fre- 

 quently only consists of a parenchyma, which at most merges gradually 

 into a tissue similar to the external cortical layer towards the exterior. 

 The three portions are the external and internal cortical layer, and the 

 liber-layer. 



Of the latter there is frequently not the smallest trace present, as, for 

 instance, in Cheiranthus Ckeiri, in a few species of Solanum, and most 

 of the Ribes, in Iledcra (?), Viburnum Lantana, Mesembryanthemum, in 

 most of the Crassuhicece, Chenopodiacece, &c. Where this liber-layer is 

 present, it consists of isolated liber-cells (as, for instance, in Cornus alba), 

 or liber-bundles (as in most Dicotyledonous trees), both being distributed 

 in the cortical parenchyma, and generally in such a manner that their 

 course corresponds accurately to that of the vascular bundles, or else it is 

 composed of a more or less accurately closed circle of liber-cells (as, for 

 instance, in Syringa, Fraxinus). Together with the liber-cells we 



* De Palmarum Structura, 12. 



